1. Field of the invention
The invention concerns a phase-locked loop demodulator.
1. Description of the prior art
The Floyd Gardner manual "Phaselock Techniques" (second edition: pages 182-183) describes a prior art phase-locked loop demodulator comprising an input bandpass filter, a phase-locked loop comprising a phase detector, a loop filter, a voltage-controlled oscillator (VCO) and a de-emphasis filter at the output of the phase-locked loop.
In a phase-locked loop of this kind any offset between the frequency (Fe) of the carrier to be demodulated and the VCO rest frequency (F.sub.vco).sub.o due to aging of the VCO, carrier drift, etc produces a static phase error at the output of the loop phase comparator causing:
degradation of the demodulator threshold, and
increased sensitivity to the phenomenon of "clicks".
Phase comparators are non-linear devices and they operate optimally with a null static phase error.
The static phase error due to a frequency offset between F.sub.e and (F.sub.VCO).sub.o is given by the equation: ##EQU1##
The value of K.sub.T is limited by:
the difficulty of implementing high-gain broadband amplifiers,
the respective sensitivities of the phase comparator and the VCO, and
the offsets and drift generated by the amplifiers.
The major drawback of a phase-locked loop of this kind is its very high sensitivity to static phase errors caused by drift affecting certain components of a receiver system and very strongly degrading the demodulator threshold.
An object of the invention is to palliate such drawbacks.